Phaselocked loop (PLL) circuits have seen widespread use wherever there is a need to synchronize a local oscillator with an independent incoming signal. A notable application may be seen in the common use of PLLs in synchronizing horizontal and vertical scan circuits in television receivers. Accuracy of synchronization whereby each and every cycle of the oscillator output signal corresponds to a like independent input signal accounts for such popularity.
Picture synchronization in a television receiver is but one application of many in modern digital transmission systems in which digital signals are frequently regenerated and wherein clock signals must be produced in order to sample and retime a received pulse stream. Applications of this type are commonplace in telephony where synchronous operation is required between circuits located at opposite ends of a telephone line.
A pulse type PLL typically has a linear pull-in telephone line range of 2.pi. radians in phase error. However, should the phase error exceed this range, the PLL will slip cycles causing an erratic operation that results in a pull-in time that is relatively long compared to the linear response time of the PLL. This problem is discussed in an article entitled, "Generalized Phase Comparators for Improved Phase-Locked Loop Acquisition", by James F. Oberst, in IEEE Transactions on Communication Technology, Vol. Com-19, No. 6, December 1971, which also describes various circuits useful in minimizing the cycle slipping problem. Although these circuits improve acquisition substantially, they do not eliminate the problem entirely and may slip a cycle before acquisition is attained.